When conducting a hydraulic fracturing operation, a hydraulic fracturing fluid is pumped into a subterranean formation under sufficient pressure to create, expand, and/or extend fractures in the formation and to thus provide enhanced recovery of the formation fluid. Hydraulic fracturing fluids typically comprise water and sand, or other proppant materials, and also commonly include various types of chemical additives. Examples of such additives include: gelling agents which assist in suspending the proppant material; crosslinkers which help to maintain fluid viscosity at increased temperatures; gel breakers which operate to break the gel suspension after the fracture is formed and the proppant is in place; friction reducers; clay inhibitors; corrosion inhibitors; scale inhibitors; acids; surfactants; antimicrobial agents; and others.
Fracturing operations have long been conducted in both low permeability and high permeability formations in order, for example, to increase the rate of production of hydrocarbon products or to increase the injection rates of water or gas injection wells. Moreover, with the introduction of slickwater fracturing procedures which use large quantities of water containing friction reducers, it is now also possible to stimulate naturally fractured shales by fracturing multiple intervals during staged treatments in horizontal wellbores. Treatment of all zones of interest in a horizontal well may require several hours to a few days to complete.
Water soluble chemical tracers have been used heretofore in hydraulic fracturing operations to trace the return of the aqueous fracturing fluid. These water soluble tracers are intended to dissolve in and flow with the aqueous fracturing fluid.
In a multistage hydraulic fracturing operation, a different chemical tracer can be added to the fracturing fluid used in each of the individual stages. After all of the fracturing stages have been completed, the fluid produced from the well is sampled and analyzed for the presence of the tracers, preferably on a periodic or continuous basis. The detection of one or more of the chemical tracers in the production fluid indicates which of the stages are flowing (i.e., the stages from which the fracturing fluids are returning).
Unfortunately, however, this procedure does not provide significant information as to which of the stages, if any, are producing formation water. Formation water is naturally occurring water which is commonly present in oil and/or gas formations. Formation water salinity levels typically range from 5 to 300 parts part thousand.
The ability to detect and evaluate the production of formation water from the individual stages of a multistage well would be greatly beneficial. Information obtained from such a method could be used to allocate the formation water produced to individual stages. Information from this procedure would also allow an operator to know if certain frac stages have contacted “wet” zones outside the intended target. The operator would then know which interval or frac needs to be shut off to prevent higher than normal formation water production. Tracer data could also help delineate between stage differences in water/oil ratios.
Consequently, a need exists for a procedure capable of (a) distinguishing formation water production from the return of the aqueous fracturing fluid, (b) detecting and determining the flow of formation water from each fracturing stage, and (c) determining the rate of formation water production from each stage or at least the comparative rates of formation water production from multiple fractured zones, particularly in horizontal wells.